Ion Pair Catalysis Research Articles

Cooperative Al(Salen)-Pyridinium Catalysts for the Asymmetric Synthesis of trans-Configured β-Lactones by [2+2]-Cyclocondensation of Acylbromides and Aldehydes: Investigation of Pyridinium Substituent Effects

The trans-selective catalytic asymmetric formation of β-lactones constitutes an attractive surrogate for anti-aldol additions. Recently, we have reported the first catalyst which is capable of forming trans-β-lactones with high enantioselectivity from aliphatic (and aromatic) aldehyde substrates by cyclocondensation with acyl bromides. In that previous study the concepts of Lewis acid and organic aprotic ion pair catalysis were combined in a salen-type catalyst molecule. Since a pyridinium residue on the salen periphery is essential for high trans- and enantioselectivity, we were interested in the question of whether substituents on the pyridinium rings could be used to further improve the catalyst efficiency, as they might have a significant impact on the effective charges within the heterocycles. In the present study we have thus compared a small library of aluminum salen/bispyridinium catalysts mainly differing in the substituents on the pyridinium residues. As one result of these studies a new catalyst was identified which offers slightly superior stereoselectivity as compared to the previously reported best catalyst. NBO calculations have revealed that the higher stereoselectivity can arguably not be explained by the variation of the effective charge.

ChemInform Abstract: Asymmetric Phase‐Transfer and Ion Pair Catalysis

AbstractReview: 111 refs.

Chiral Organic Contact Ion Pairs in Metal-Free Catalytic Asymmetric Allylic Substitutions

Chiral contact ion-pair catalysis with particular focus on metal-free processes is gaining in interest. As a result, new perspectives are opened, and highly stereoselective transformations, traditionally performed under metal catalysis, can be realized. Herein, we report the development of an unprecedented asymmetric Brønsted acid-catalyzed allylic alkylation. The concept relies on chiral contact ion-pair catalysis, in which the chiral organic counteranion of an allylic carbocation induces high enantioselectivities and allows access to biologically relevant chromenes in good yields and with excellent enantioselection.

ChemInform Abstract: Catalytic Asymmetric Synthesis of trans‐Configured β‐Lactones: Cooperation of Lewis Acid and Ion Pair Catalysis.

AbstractThe present, efficient protocol for catalytic asymmetric cyclocondensation of acyl halides with aldehydes is based on combination of Lewis acid and organic aprotic ion pair catalysis in an single catalyst system.

Catalytic Asymmetric Synthesis of trans‐Configured β‐Lactones: Cooperation of Lewis Acid and Ion Pair Catalysis

The development of the first trans-selective catalytic asymmetric [2+2] cyclocondensation of acyl halides with aliphatic aldehydes furnishing 3,4-disubstituted beta-lactones is described. This work made use of a new strategy within the context of asymmetric dual activation catalysis: it combines the concepts of Lewis acid and organic aprotic ion pair catalysis in a single catalyst system. The methodology could also be applied to aromatic aldehydes and offers broad applicability (29 examples). The utility was further demonstrated by nucleophilic ring-opening reactions that provide highly enantiomerically enriched anti-aldol products.

ChemInform Abstract: New Developments in Enantioselective Bronsted Acid Catalysis: Chiral Ion Pair Catalysis and Beyond

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Contact Ion Pair Directed Lewis Acid Catalysis: Asymmetric Synthesis of trans‐Configured β‐Lactones

Opposites attract. A novel concept for dual-activation catalysis takes advantage of the cooperative action of aprotic contact ion pair catalysis and Lewis acid catalysis. This strategy was used for the first trans-selective catalytic asymmetric [2+2] cyclocondensation of acyl halides with aliphatic aldehydes, furnishing trans-configured 3,4-disubstituted β-lactones with high stereoselectivity.

Electrostatic properties in the catalytic site of papain: A possible regulatory mechanism for the reactivity of the ion pair.

We present an analysis of the electrostatic properties in the catalytic site of papain (EC 3.4.22.2), an archetype enzyme of the C1 cysteine proteinase family, and we investigate their possible role in the formation, stabilization and regulation of the Cys25((-))...His159((+)) catalytic ion pair. The electrostatic properties were computed using a reassociation method based in multicentered multipolar expansions obtained from ab initio quantum calculations of overlapping protein fragments. Solvent effects were introduced by coupling the use of multicentered multipolar expansions to two continuum boundary element methods to solve the Poisson and the linearized Poisson-Boltzmann equations. The electrostatic profile found in the proton transfer region of papain showed that this enzyme has a well-defined electrostatic environment to favor the formation and stabilization of the catalytic ion pair. The papain catalytic site electrostatic profile can be considered as an electrostatic fingerprint of the papain family with the following characteristics: (i) the presence of a net electric field highly aligned in the (Cys25)-SG-->(His159)-ND1 direction; (ii) the electrostatic profile has a saddle-point character; (iii) it is basically a local environmental effect. Furthermore, our analysis describes a possible regulatory mechanism (the E(SG-->ND1) attenuation effect) controlling the ion pair reactivity and permits to infer the Asp57 acidic residue as the most probable candidate to act as the electrostatic modulator.

A Comparison of the Effects of Ion Pairing and Blocking Additives on Electrodeposition of Zinc and Cadmium

A comparison was made of the effects of benzyl alcohol and tetraalkylammonium ions on current efficiencies for deposition of cadmium and zinc. Benzyl alcohol, which blocked hydrogen evolution and lowered the interfacial dielectric constant, increased current efficiencies for the deposition of zinc and cadmium. Tetraalkylammonium salts, which had been shown to behave consistently as ion pairing catalysts for deposition of cadmium from chloride systems, were found to be inconsistent catalysts for deposition of zinc from chloride media. This difference is attributed to the fact that formation constants of the cadmium chloride complexes are much larger than formation constants for the zinc complex. Because zinc‐cyanide formation constants are higher than zinc‐chloride formation constants, the ion pairing catalysis of the tetramethylammonium ion in the presence of cyanide, with and without benzyl alcohol, was higher in the cyanide system, leading to higher current efficiencies. In the presence of benzyl alcohol, when tetrabutylammonium ions were used they did not catalyze deposition of the metal as well as tetramethylammonium ions. This difference in rate is ascribed to the presence at the cathode of a benzyl alcohol film with pores through which the smaller tetramethylammonium ion‐pair can pass more easily than can the larger tetrabutylammonium ion‐pair.

Interaction of aspartic acid-104 and proline-287 with the active site of m-calpain.

In an ongoing study of the mechanisms of calpain catalysis and Ca(2+)-induced activation, the effects of Asp-104-->Ser and Pro-287-->Ser large subunit mutations on m-calpain activity, the pH-activity profile, Ca(2+)-sensitivity, and autolysis were measured. The importance of these positions was suggested by sequence comparisons between the calpain and papain families of cysteine proteinases. Asp-104 is adjacent to the active-site Cys-105, and Pro-287 is adjacent to the active-site Asn-286 and probably to the active-site His-262; both Asp-104 and Pro-287 are absolutely conserved in the known calpains, but are replaced by highly conserved serine residues in the papains. The single mutants had approx. 10-15% of wild-type activity, due mainly to a decrease in kcat, since Km was only slightly increased. The Pro-287-->Ser mutation appeared to cause a local perturbation of the catalytic Cys-105/His-262 catalytic ion pair, reducing its efficiency without major effect on the conformation and stability of the enzyme. The Asp-104-->Ser mutation caused a marked narrowing of the pH-activity curve, a 9-fold increase in Ca2+ requirement, and an acceleration of autolysis, when compared with the wild-type enzyme. The results indicated that Asp-104 alters the nature of its interaction with the catalytic ion pair during Ca(2+)-induced conformational change in calpain. This interaction may be direct or indirect, but is important in activation of the enzyme.

New chiral crown ethers derived from camphor and their application to asymmetric Michael addition. First attempts to rationalize enantioselection by AM1 and AMBER calculations

The synthesis of novel optically active crown ethers derived from (1 R)-(+)-camphor is described. The mechanism of their catalytic effect upon the Michael addition of phenylacetate to acrylate is discussed in terms of kinetic vs. thermodynamic control in the formation of the catalytic ion-pair complexes. The relative basicity of the complexes formed between the alkaline metals and the unprotonated chiral crown ethers plays an important role in the sterochemical outcome. AMBER and AM1 calculations support that the reaction in which the best e.e. (83%) is obtained proceeds under kinetic control.

Ion pair catalysis of the autooxidative cleavage of benzoin in emulsion systems

AbstractA study was made of the autooxidation of benzoin in organic‐aqueous sodium hydroxide emulsions in the presence of several quaternary ammonium salts. The rate of the reaction was found to be accelerated by these salts, probably by ion pair extraction. In pyridine and acetonitrile systems the reaction was almost exclusively the formation of benzoate. Neither benzil nor benzaldehyde were intermediates in the oxidation process. The rate law for the reaction was found to be which led to the conclusion that the rate‐determining step in the catalyzed pathway was the reaction of oxygen with the ion pair: magnified image

Ion pair catalysis of the autooxidation of pyridine and benzil in emulsion systems

AbstractIt was found that the rates of absorption of oxygen by pyridine–aqueous sodium hydroxide emulsions and the same emulsions containing benzil were catalyzed by the addition of quaternary salts and followed the same rate law: . It was concluded that the autooxidation of benzil in pyridine‐aqeuous sodium hydroxide emulsions has as its rate‐determining step a step in the autooxidation of pyridine. Possibly the superoxide formed in the autooxidation of pyridine is the oxidizing agent in the oxidation of benzil.

Contribution of phase transfer to resolve the problem of separation between the catalyst and the product in the catalytic carbonylation of halides.

Study of carbonylation of benzyl bromide in a biphasic liquid-liquid system shows that the catalytic ion pair Bu 4N +Co(CO) 4 − stays in the organic layer and that the carbonylated product C 6H 5CH 2CO 2 − stays in the aqueous one.